The present invention relates to a black ink composition and an ink set, and further relates to a recording method using the same, recorded matter obtained using the same, and an ink jet recording device using the same.
In the case where an image is formed by an ink jet recording method or the like, there may be used an ink set comprising a light black ink for the purpose of obtaining a high quality image. For example, to a four-color ink set comprising a black ink, a cyan ink, a magenta ink and a yellow ink (or a six-color ink set further comprising a light cyan ink and a light magenta ink added to the four-color ink set) may be added a light black ink to make a five-color ink set (or seven-color ink set). The light black ink has a lower carbon black concentration than that of the black ink and is used for the purpose of improving the color reproducibility with respect to dark color such as shadow area or improving the gradation of gray.
However, since the light black ink (i.e., thin black ink) has a low pigment content, the resulting color hue is tinted rather than anchromatically colored which state is originally necessary. In other words, the light black ink deviates from (a*, b*) of (0, 0) in the L*a*b* space. In some detail, the light black ink is tinted with yellow. In order to modify the yellowing property of carbon black, it is necessary that a color compensator (i.e., pigment) complementary to yellow be used. In the case where the pigment complementary to yellow is selected from organic pigments, it is necessary that discoloration by air oxidation or light-resistance be taken into account.
For example, U.S. Pat. No. 5,803,958 discloses a black pigment ink composition comprising a cyan pigment and a magenta pigment in addition to a carbon black. The above cited U.S. Pat. No. 5,803,958 discloses a black ink composition comprising Pigment Blue 15:3 as a cyan pigment and Pigment Red 122 as a magenta pigment. The use of this black ink composition as a light black ink composition makes it possible to suppress the foregoing yellowing property. However, the foregoing Pigment Blue 15:3 and Pigment Red 122 were disadvantageous in that the yellowability of carbon black may be emphasized with time contrarily.
An object of the present invention is to provide means that can eliminate the defects of light black ink compositions according to the related art technique even stably with time to modify the yellowing property of carbon black.
Other objects and effects of the present invention will become apparent from the following description.
As a result of the extensive studies made by the present inventors, it was found that the use of a specific blue pigment as a complementary color makes it possible to obtain a black ink composition which exhibits a good tint and an excellent light-resistance and can withstand discoloration due to air oxidation. The black ink composition can be used as a light black ink composition to eliminate the defects of the related art technique even stably with time. The black ink composition can also be suitably used as usual black ink (ink having a high carbon black concentration).
The inventors further found that an ink set comprising a plurality of light black ink compositions wherein the comparison of any two black ink compositions contained therein shows that the ratio of the concentration of the specific pigment to the concentration of carbon black in the black ink composition having a lower carbon black concentration (e.g., light black ink composition) is higher than the ratio of the concentration of the specific pigment to the concentration of carbon black in the black ink composition having a higher carbon black concentration (e.g., black ink composition) can form a gray scale which is modified in yellowing property even stably with time.
The invention is based on the above-described findings.
That is, the above-described objects of the present invention have been achieved by providing the followings.
1) A black ink composition comprising a carbon black and a pigment represented by the following general formula 
wherein p represents an integer of from 0 to 3.
2) The black ink composition according to item 1) above, which is aqueous.
3) The black ink composition according to item 1 or 2 above, further comprising at least one of an anionic surfactant, a nonionic surfactant and an amphoteric surfactant.
4) The black ink composition according to item 3) above, wherein said nonionic surfactant is an acetylene glycol-based surfactant.
5) The black ink composition according to any one of items 1) to 4) above, further comprising a water-soluble organic solvent.
6) An ink set comprising, as a black ink composition or a light black ink composition, a black ink composition comprising a carbon black and a pigment represented by the following general formula (1): 
wherein p represents an integer of from 0 to 3.
7) The ink set according to item 6) above, comprising a plurality of black ink compositions each comprising a carbon black and a pigment of formula (1),
wherein the plurality of black ink compositions have different carbon black concentrations from one another, and
wherein any two of the plurality of black ink compositions satisfy the following relationship (2):
P1/K1 greater than P2/K2 xe2x80x83xe2x80x83(2) 
wherein P1 and K1 represent the concentrations of the pigment of formula (1) and of the carbon black, respectively, in the black ink composition having a lower carbon black concentration between the two black ink compositions, and P2 and K2 represent the concentrations of the pigment of formula (1) and of the carbon black, respectively, in the black ink composition having a higher carbon black concentration between the two black ink compositions.
8) The ink set according to item 7) above, wherein said black ink compositions comprising a carbon black and a pigment of formula (1) includes a light black ink composition and a black ink composition.
9) The ink set according to any one of items 6) to 8) above, which is aqueous.
10) A recording method comprising:
ejecting a droplet of an ink composition so that said droplet is attached to a recording medium to effect printing,
wherein said ink composition is a black ink composition according to any one of items 1) to 5) above or is a constituent of an ink set according to any one of items 6) to 9) above.
11) Recorded matter obtained by printing in accordance with a recording method according to item 10) above.
12) An ink jet recording device comprising an electrostrictive element capable of oscillating according to electrical signal and being arranged such that the oscillation of said electrostrictive element causes ejection of a black ink composition,
wherein said ink composition is a black ink composition according to any one of items 1) to 5) above or is a constituent of an ink set according to any one of items 6) to 9) above.